Shot peening treatment has been known as a means for improving mechanical properties of a casting (see, for example, Patent Document 1). The shot peening treatment is a cold working carried out as a post-step for a casting, and is a technology according to which the surface layer of a casting is subjected to plastic deformation by projecting fine particles at a high speed to generate residual compressive stress, whereby mechanical strength of the casting can be improved.
However, since this shot peening treatment can treat only the surface layer above the surface of a casting, it does not have the effect to remove internal defects which may be present above the surface of a casting, and has limit in improving mechanical properties of a casting and may rather induce deterioration in properties of the surface layer of a casting.
Therefore, the applicant has proposed a method for hardening of a casting as a new alternative means for the shot peening (see Patent Document 2). This is a means to form a dense layer at the surface side of the casting thicker than the plastic deformation layer formed by shot peening, and according to the method for hardening of castings disclosed in Patent Document 2, desired mechanical properties can be imparted to a casting. This method for hardening of a casting is a means to form a dense layer by oscillating the castings together with a hardening material to collide the hardening material against the surface of a casting. Therefore, a oscillating device is necessary for performing this means.
The oscillating device is disclosed in Patent Document 2, and, besides, the applicant has made a new proposal in Patent Document 3. Moreover, as prior art documents relating to oscillating, there are known Patent Document 4 disclosing a barrel polishing technology proposed by the applicant, and Patent Document 5 and Patent Document 6 presenting a technology to remove core sand.    Patent Document 1: JP-B-8-11366    Patent Document 2: JP-A-2004-322112    Patent Document 3: JP-A-2004-174604    Patent Document 4: JP-A-4-289069    Patent Document 5: JP-A-10-286665    Patent Document 6: JP-A-61-9961